It is well known that an aromatic polyimide membrane or film can be produced from a dope solution of a polyamic acid, which solution has been prepared by the polymerization reaction of an aromatic tetracarboxylic acid component with an aromatic diamine component in an organic polar solvent, by spreading the dope solution on a smooth surface of a film-forming device to form a thin layer of the dope solution and, then, by evaporating the solvent from the dope solution layer while converting the polyamic acid to the corresponding polyimide, so as to form a thin solid film of the polyimide. This process has been actually utilized in industry.
However, it is also known that the conventional dense membrane consisting of the aromatic polyimide exhibits a remarkably poor permeating rate of a gas or liquid. Therefore, the conventional aromatic polyimide membrane can not be used as a separator for a gas mixture or a liquid mixture or as a concentrator of a gas or liquid, unless the membrane is made very thin. The conventional process can not provide a thin porous aromatic polyimide membrane having a satisfactory permeating rate for a gas or liquid.
Under the above-mentioned circumstances, it was strongly desired to provide a new process for producing an aromatic polyimide membrane from a polyamic acid said membrane having a porous structure and exhibiting an enhanced permeating rate of a gas or liquid therethrough which is useful as a separator of a gas or liquid mixture or as a concentrator of a gas or liquid.